1. Field of the Invention
This invention relates generally to mixing and measuring apparatus and more particularly to an improved control system and apparatus for mixing a plurality of dry, granular, slurry and/or powdered materials being fed into a blender on a continuous basis by maintaining precise control over the ratios of feed rates. An extension of the basic invention permits the added advantage of mixing liquid materials.
2. Description of the Prior Art
Mixing and measuring apparatus are known in which a plurality of granular materials in a predetermined proportion purportedly can be mixed, however, much difficulty has been encountered in accurately proportioning the mix components. Much has depended on the skill of the individual craftsman in operating the various devices claiming the ability to blend disparate materials.
In the plastics industry, virgin plastic, in pellet form, with color dye in pellet or powder form, is blended on a continuous basis. A number of schemes have been used, however, the problems encountered are typical of problems encountered in other applications as well.
Color concentrate may be fed in both the pellet and powder form. Powder is usually used in applications in which the end product is one color and one color only. In this application, the whole facility becomes the color of the powder dye.
In pelletized dye applications, the dye concentrate must usually be prepared, as for example, a small amount of plastic is introduced into the dye and the mixture is then extruded. The extruded product is then chopped up to form the pellets. The color is re-released when the plastic material is further processed, for example, by melting down and running through the auger of an extruding machine. It is easier to change colors of the end product when the pellet-dye process is used since the whole facility does not take on the color of a dye.
Some of the prior art problems have been attacked through the device of feeding a colorant into an auger type extrusion machine at a predetermined rate. A vibrator is commonly used to control the rate of colorant fed, for example, three unit weights of colorant per eighty-five unit weights of raw plastic pellets. Thus the ratio 3/85 is set up in the units noted.
The vibrator, mentioned above, is used in many applications in conjunction with a tray or trough and a control for controlling the speed of vibration. Remaining with the plastics example for the moment, most extrusion machines of the auger type are rated with respect to the amount of material delivered by the auger to an extrusion head per unit time. Colorant may be fed into the auger at a convenient location at a rate determined by the speed of vibration of the colorant-feeding vibrating tray. In our numerical example above, it is necessary to set our rate of vibration of the colorant-feeding vibrating tray so that it will supply three eighty-fifths (3/85) of the amount of colorant with respect to the rated operating capacity of the auger machine.
Typically, the feed rates of the color and of the virgin plastic are controlled by rheostats connected in series with their respective vibrators. Materials are thus taken from bins and fed into a blender mechanism. Adjustment of the rheostat controls the A.C. voltage applied to the vibrator; thus, the rate of feed of materials is controlled. Each vibrator, however, has its own internal impedance which, in operation, must be added to the resistance of the rheostat thus making the control non-linear. Because of this inherent non-linearity, mechanical ganging of the rheostats is of no practical value. Gross errors thus result in attempting to set the virgin plastic-to-color material ratios.
The feed of the virgin plastic, as described above, is effected in general by means of a rheostat-controlled vibrator, while the feed of the color material may be controlled by means of an auger driven by a D.C. motor. It has been observed that though the control by means of a rheostat of a vibrator is non-linear, the control of a D.C. auger drive approaches linearity fairly closely. Slightly better control of the color feed rate to match the virgin plastic feed rate may thus be realized. A great disadvantage encountered, however, is the fact that there are two independent controls with which an operator must work thus making the system heavily dependent upon the skill and experience of the operator in matching the flow rates.
The use of A.C. regulated power supplies offers a practical alternative to the use of rheostats as described above. However, in order to obtain a linear vibrator output with respect to applied voltage, it is necessary to generate a smooth sinusoidal wave-form. Difficulties in generating and maintaining a smooth sine wave of voltage under load conditions are well known. Again, it is impractical to gang controls together.
Another interesting device blends a precise amount of one material to a measured batch of another material. This batch process is in general use in the plastics industry and in other industries in which precision control of the individual components of a mix is required. Typically, in the batch process, individual components of the mix are fed sequentially into the weighing hopper of a scale. When the desired weight of one component has been loaded onto the weighing hopper, the feeding operation is interrupted and the next mix component is then loaded into the weighing hopper until the scale indicates that the proper weight has been added. This process is repeated until all of the mix components have been added to the weighing hopper in their respective required amounts as indicated by the weighing scale. After all of the components of the mix have been loaded into the weighing hopper, the contents of the weighing hopper are introduced into a blending drum where the mix is tumbled until the mixing process is deemed complete.
It is obvious that a method of this type inherits all the disadvantages of the batch type mixing, for example, dependence on the skill of the operator, precise timing required, tolerances, overruns, etc. The process is inherently slow, requiring as it does, that each component be weighed out meticulously and sequentially. After the weighing process, the batch is usually dropped by gravity feed into a baffled mixing drun where it is blended, generally by means of rotation. It is not generally practical to carry out the blending operation simultaneously with the weighing step since vibrations fed into the system by the rotating blending drum disturb the accuracy of the weighing scale.
Another disadvantage of the batch system involves the actual mixing. Since the mixing is done empirically, there is no certain way by which it may be known just how uniform and homogeneous a mixture has become after being resident in the mixing drum for this empirical time interval. Components making up a relatively small amount of the total mix may tumble about in little wads of concentration and never truly blend uniformly with the balance of the mix. Thoroughness of blending is thus also a multi-variable function of mixing drum design, speed of rotation or agitation, total number of cycles of rotation or agitation, nature of the materials being blended, etc.
One of the most troublesome disadvantages is one involving tolerances. Trace quantities in particular, pose a serious problem for the batch process. Weighing scales typically have an accuracy of .+-.0.1% of full scale reading. For purposes of illustration, assume a total batch weight of 2/3 of scale capacity. Assume further a trace element of 1% of the weight of the batch. The error inherent in the batch system in metering this small quantity is .+-.15%. If an error of this magnitude cannot be tolerated, an independent and more precise scale system must be utilized in addition to the original weighing system.
Another troublesome area is called out by the term "overruns". In the batch method of mixing, a predetermined amount of the mixture must be produced. For example, if 1.5 batches are required, two batches must be mixed and the surplus either stored, discarded, or disposed of in some way. The point is, 1.5 batches cannot be produced ab initio by the batch method.
One method of feeding a dry particulate material to be mixed from a bin is to use a vibrating tray as has been noted. It can be shown that the amount of material fed is directly proportional to the voltage applied to the vibrator. Stated in the succinct symbolism of mathematics, we have: EQU M=K.times.V
where:
M is the amount of material fed; PA1 K is a proportionality constant; PA1 V is the applied voltage. PA1 M is the amount of material fed; PA1 K is a proportionality constant; PA1 V is an applied voltage,
Almost all vibrator systems of the kind considered here incorporate some form of rectifying device applying alternating electric energy to a solenoid coil so that on the half cycle that the vibrator solenoid coil is energized, the vibrator tray is pushed forward by the solenoid. On the half cycle that it is deenergized, a spring snaps the tray back thus leaving the materials advanced on the tray. Vibrating systems are well known in many arts and their application, for example, to the plastics industry has been indicated.
Many different mixing and measuring devices have been employed in the attempts to solve the problems presented. Most have either presented new problems or only partially solved the problems presented or both. Most of these devices have thus met special needs as presented by specific problems and have, therefore, served narrow purposes. These prior art devices, among other disadvantages, have caused unacceptable amounts of one particulate material to be mixed with another as a result of tolerance buildup in the measuring method employed. A number of these prior art devices have been unreliable and unpredictable in operation under continued use and have been expensive and complicated to manufacture and maintain. Some of these prior art devices have been described in the following listed patents that were brought to the attention of the applicant through a novelty search conducted in the U.S. Patent and Trademark Office.
______________________________________ PATENT NO. TITLE INVENTOR ______________________________________ 3,154,808 Continuous Internal Ahlefeld et al Stiff-Gel Mixer 3,182,969 Blending Apparatus Rupp 3,228,065 Device for Feeding Raw Cournoyer et al Material to Plastic Extrusion Machines 3,231,243 Apparatus for Blending Bulk Armstrong Raw Materials with Colorant 4,021,022 Pigment Metering and Satterfield et al Mixing Apparatus ______________________________________
While reference patents have been drawn very heavily from the plastics industry, obvious application is immediately apparent in other disciplines, for example, food mixing, animal feed preparation especially with respect to vitamin and hormone addition.
It would thus be a great advantage to the art to provide for the mixing of materials in preselected ratios.
A further great advantage would be to provide for repeatability in the selection of given ratios among materials to be mixed.
A still further advantage would be to provide for improved mixing means so as to control the quantity of a mix so provided thus preventing overruns.
Yet another advantage would be to provide an apparatus in which the accuracies of the ratios of the materials to be mixed would be independent of operator skill.
An additional advantage would be the provision of an apparatus for mixing materials in which the requirement for precise timing is eliminated.
A further great advantage would be to provide a system accomplishing the other above-noted advantages that is simple and easy to operate and economical to manufacture, install and maintain.